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Lifting

Posted on July 17, 2011, in Programming

Below is a compileable Scala source file. If you read it from top to bottom, it may help with some insights regarding applicative functors. It was partially inspired by Eric’s rendition of The Essence of the Iterator Pattern.

trait Lift[F[_]] {
  // Spot the pattern in these type signatures
  // of increasing arity.

  def lift0[A]:
    A => F[A]

  def lift1[A, B]:
    (A => B) => (F[A] => F[B])

  def lift2[A, B, C]:
    (A => B => C) => (F[A] => F[B] => F[C])

  def lift3[A, B, C, D]:
    (A => B => C => D) => (F[A] => F[B] => F[C] => F[D])

  // ... and so on

  // The relationship between lift<n> and lift<n-1>
  // can be given by a function,

  def ap[A, B]:
    F[A => B] => (F[A] => F[B])
}

trait LiftImpl[F[_]] extends Lift[F] {
  // Each lift function uses
  // the previous lift function and ap.

  def lift1[A, B]:
    (A => B) => (F[A] => F[B])
    = ap compose lift0

  def lift2[A, B, C]:
    (A => B => C) => (F[A] => F[B] => F[C])
    = f => ap compose lift1(f)

  def lift3[A, B, C, D]:
    (A => B => C => D) => (F[A] => F[B] => F[C] => F[D])
    = f => a => ap compose lift2(f)(a)
}

// Notes
// * lift0 is often called: unit, return, pure, point, η
// * lift1 is often called: fmap, map, ∘
// * lift<n> is often called: liftA<n>, liftM<n>

All that is left to do is to implement the LiftImpl trait! You can do this by implementing the ap and lift0 functions.

Examples of implementations that I know will work out if you try to implement them:

Those last couple are a bit funky, but a lot of that is syntax noise rather than anything too complicated. Fill out the body of those classes!